A number of types of EM digitizers or digitizer tablets are known in the art. In general, a digitizer tablet provides an indication of a position of a probe with respect to a surface of a planar sensor grid. One type of digitizer tablet operates by sensing a magnetic field and includes two sets of array conductors, with one set being orthogonally oriented to the other set. In this type of system the probe is driven with an AC signal to cause an oscillating magnetic field which induces signals in the array conductors. The signals in the array conductors are sensed and compared so as to determine the position of the probe in two dimensions, relative to the surface of the tablet.
Other types of digitizer tablets are also known, such as tablets that drive the array conductors and sense the field with the probe. Tablets that operate with electric fields and resistive bridges are also known. Tablets that include magnetostrictive elements are also well represented in the prior art.
The magnetic field sensing tablet is of particular interest herein. More specifically, this type of tablet measures a mutual inductance of a pair of coils. One of the pair of coils is located within the tablet and the other coil is located within the probe, also referred to herein as a stylus or pen. The pen may be physically coupled to the tablet through one or more wires, referred to as a "tethered" pen, or may be physically decoupled from the tablet as an "untethered" pen. The untethered pen embodiment is preferred in that a more natural user interface is provided.
Many tablet sensor grid arrangements consist of sets of identical, elongated coils arranged along the tablet x-axis and y-axis. One conventional method of estimating pen position is to measure the signal strengths in the coils near the pen and to estimate the location of a peak signal strength along each dimension. This is equivalent to finding a zero-crossing of the signal derivative in each dimension. Since all calculations are performed on the derivative, it is convenient to refer to measurements of the derivative. These measurements are conventionally obtained by taking the differences of signals in adjacent coils from the coil array.
A problem associated with conventional digitizer tablets is in accurately predicting pen position from the signals recovered from the sensor coils. In this regard it is known that pen tilt, or orientation with respect to a normal to the surface of the tablet, influences a zero crossing estimate of the coil signals and, hence, an estimate of pen position. Aside from the effect of tilt on the position estimate, the pen tilt information itself, in conjunction with x-y positional information, may be of considerable value to some users of digitizer tablets, such as automatic on-line handwriting recognition systems.
The following chronologically ordered U.S. patents are representative of a significant number of U.S. patents that have been issued in this technical area.
In U.S. Pat. No. 3,904,822, issued Sep. 9, 1975, entitled "Absolute Position Determining System Using Free Stylus" to Kamm et al. there is described orthogonal conductor grids that are energized with time spaced current pulses. A discussion of stylus tilt is provided in reference to FIG. 4 and circuitry is disclosed for generating an output signal and an estimated zero-crossing time.
In U.S. Pat. No. 4,185,165, issued Jan. 22, 1980, entitled "Low Noise System and Method for Sequentially Sensing Induced Signals in Digitizer Grid Conductors" to Fencl there is described a high signal-to-noise ratio digitizing system having a pen with a coil disposed around a nose piece. A grid of conductors includes a group of parallel spaced conductors oriented in X and Y directions. A 96 KHz alternating current voltage is induced in the X and Y conductors by the pen coil and a differential amplifier is connected to the top and the bottom of a single coil through multiplexers.
In U.S. Pat. No. 4,477,877, issued Oct. 15, 1984, entitled "Coordinate Determining Device" to Nakamura et al. there is described a coordinate determining device that includes means for compensating for an error of a computed position that is caused by an inclination of a coordinate indicator relative to a tablet. This technique uses detected secondary peak voltage values within sensing lines that are remote from a sensing line having a maximum voltage value.
U.S. Pat. No. 4,552,991, issued Nov. 12, 1985, entitled "Absolute Position Coordinate Determining Device Employing a Single Phase Difference Measurement to Determine Cursor Position" to Hulls there is described a two dimensional digitizing system that includes at least two conducting grid systems. Each grid system has a number of windings with a first pitch distance and an equal number of windings with a second pitch distance that differs from the first pitch distance by a small increment. This technique is said to enable both coarse and fine position measurements.
In U.S. Pat. No. 4,717,793 issued Jan. 5, 1988, entitled "Graphic Input Device" to Kobayashi there is described a graphic input device having envelope detection that accommodates a range of stylus or pen inclinations. FIG. 13B illustrates how the tilting of a pen causes an intense magnetic field to interlink those conductor loops toward which the pen is tilted. As a result, a significant peak is said to appear in a signal, as shown in FIG. 15B.
In U.S. Pat. No. 4,928,256, issued May 22, 1990, entitled "Digitizer for Position Sensing", to Parnell et al. there is described apparatus and method for determining the position, tilt, and offset of a pointer with respect to a grid of parallel conductors. This method involves storing information that is functionally related to a characteristic shape of a response curve, and mathematically fitting measured responses to a functional expression that represents the stored characteristic shape so as to determine the positional coordinates of the pointer.
In U.S. Pat. No. 4,939,318, issued Jul. 3, 1990, entitled "Digitizer Pen Tilt Correction Employing Wires Near the Data Point" to Watson et al. there is described a digitizer tablet system that employs a method of compensating a calculation of a position of a pen tip. The method includes calculating an approximation to the pen tilt and adding a preestablished constant that is multiplied by the tilt.
In U.S. Pat. No. 4,990,726, issued Feb. 5, 1991, entitled "Digitized Controller for Position Locator" to Lasley there is described a digitizer tablet that determines a pair of sample values from wires located under opposite sides of a pointing device and interpolates between the pair of sampled values to determine the location of the pointing device.
What is not taught by this prior art, and what is thus an object of the invention to provide, is an estimator of both pen position and tilt that employs weighted coil signal measurements.
A further object of the invention is to provide an estimator of both pen position and tilt that employs coil signal measurements that are weighted in accordance with a gaussian weighting function.